The introduction of desired agents into specific target cells has been a challenge to scientists for a long time. The challenge of specific targeting of agents is to get an adequate amount of the agent or the correct agent to the target cells of an organism without providing too much exposure of the rest of the organism. A very desired target for delivery of specific agents is the immune system. The immune system is a complex response system of the body that involves many different kinds of cells that have differing activities. Activation of one portion of the immune system usually causes a variety of responses due to unwanted activation of other related portions of the system. Currently, there are no satisfactory methods or compositions for producing a specifically desired response by targeting the specific components of the immune system.
The immune system is a complex interactive system of the body that involves a wide variety of components, including cells, and cellular factors, which interact with stimuli from both inside the body and outside the body. Aside from its direct action, the immune system's response is also influenced by other systems of the body including the nervous, respiratory, circulatory, and digestive systems.
One of the better-known aspects of the immune system is its ability to respond to foreign antigens presented by invading organisms, cellular changes within the body, or from vaccination. Some of the first kinds of cells that respond to such activation of the immune system are phagocytes and natural killer cells. Phagocytes include among other cells, monocytes, macrophages, and polymorphonuclear neutrophils. These cells generally bind to the foreign antigen, internalize it and often times destroy it. They also produce soluble molecules that mediate other immune responses, such as inflammatory responses. Natural killer cells can recognize and destroy certain virally-infected embryonic and tumor cells. Other factors of the immune response include complement pathways, which are capable of responding independently to foreign antigens or acting in concert with cells or antibodies.
One of the aspects of the immune system that is important for vaccination is the specific response of the immune system to a particular pathogen or foreign antigen. Part of the response includes the establishment of “memory” for that foreign antigen. Upon a secondary exposure, the memory function allows for a quicker and generally greater response to the foreign antigen. Lymphocytes in concert with other cells and factors play a major role in both the memory function and the response.
Generally, it is thought that the response to antigens involves both humoral responses and cellular responses. Humoral immune responses are mediated by non-cellular factors that are released by cells and which may or may not be found free in the plasma or intracellular fluids. A major component of a humoral response of the immune system is mediated by antibodies produced by B lymphocytes. Cell-mediated immune responses result from the interactions of cells, including antigen presenting cells and B lymphocytes (B cells) and T lymphocytes (T cells).
One of the most widely employed aspects of the immune response capabilities is the production of monoclonal antibodies. The advent of monoclonal antibody (Mab) technology in the mid 1970s provided a valuable new therapeutic and diagnostic tool. For the first time, researchers and clinicians had access to unlimited quantities of uniform antibodies capable of binding to a predetermined antigenic site and having various immunological effector functions. Currently, the techniques for production of monoclonal antibodies are well known in the art.
These monoclonal antibodies are thought to hold great promise in medicine and diagnostics. Unfortunately, the development of therapeutic products based on these proteins has been limited because of problems that are inherent in monoclonal antibody therapy. For example, most monoclonal antibodies are mouse derived and, thus, do not fix human complement well. They also lack other important immunoglobulin functional characteristics when used in humans.
The biggest drawback to the use of monoclonal antibodies is the fact that nonhuman monoclonal antibodies are immunogenic when injected into a human patient. After injection of a foreign antibody, the immune response mounted by a patient can be quite strong. The immune response causes the quick elimination of the foreign antibody, essentially eliminating the antibody's therapeutic utility after an initial treatment. Unfortunately, once the immune system is primed to respond to foreign antibodies, later treatments with the same or different nonhuman antibodies can be ineffective or even dangerous.
Mice can be readily immunized with foreign antigens to produce a broad spectrum of high affinity antibodies. However, the introduction of murine antibodies into humans results in the production of a human-anti-mouse antibody (HAMA) response due to the presentation of a mouse antibody in the human body. Use of murine antibodies in a patient is generally limited to a term of days or weeks. Longer treatment periods may result in anaphylaxis. Moreover, once HAMA has developed in a patient, it often prevents the future use of murine antibodies for diagnostic or therapeutic purposes.
To overcome the problem of HAMA response, researchers have attempted several approaches to modify nonhuman antibodies, to make them human-like. These approaches include mouse/human chimers, humanization, and primatization. Early work in making more human-like antibodies used combined rabbit and human antibodies. The protein subunits of antibodies, rabbit Fab fragments and human Fc fragments, were joined through protein disulfide bonds to form new, artificial protein molecules or chimeric antibodies.
Recombinant molecular biological techniques have been used to create chimeric antibodies. Recombinant DNA technology was used to construct a gene fusion between DNA sequences encoding mouse antibody variable light and heavy chain domains and human antibody light chain (LC) and heavy chain (HC) constant domains to permit expression of chimeric antibodies. These chimeric antibodies contain a large number of nonhuman amino acid sequences and are immunogenic to humans. Patients exposed to these chimeric antibodies produce human-anti-chimera antibodies (HACA). HACA is directed against the murine V region and can also be directed against the novel V-region/C-region (constant region) junctions present in recombinant chimeric antibodies.
To overcome some of the limitations presented by the immunogenicity of chimeric antibodies, molecular biology techniques are used to created humanized or reshaped antibodies. The DNA sequences encoding the antigen binding portions or complementarity determining regions (CDRs) of murine monoclonal antibodies are grafted, by molecular means, on the DNA sequences encoding the frameworks of human antibody heavy and light chains. The humanized Mabs contain a larger percentage of human antibody sequences than do chimeric Mabs. The end product, which comprises approximately 90% human antibody and 10% mouse antibody, contains a mouse binding-site on a human antibody. It also contains certain amino acid substitutions from the mouse Mab into the framework of the humanized Mab in order to retain the correct shape, and thus, binding affinity for the target antigen.
In practice, simply substituting murine CDRs for human CDRs is not sufficient to generate efficacious humanized antibodies retaining the specificity of the original murine antibody. There is an additional requirement for the inclusion of a small number of critical murine antibody residues in the human variable region. The identity of these residues depends upon the structure of both the original murine antibody and the acceptor human antibody. It is the presence of these murine antibody residues that helps create a HACA response in the patient, leading to rapid clearance of the monoclonal antibodies and the fear of anaphylaxis.
Another technique, called resurfacing technology, is used for humanizing mouse antibodies. Resurfacing involves replacing the mouse antibody surface with a human antibody surface in a process that is faster and more efficient than other humanization techniques. This technique provides a method of redesigning murine monoclonal antibodies to resemble human antibodies by humanizing only those amino acids that are accessible at the surface of the V-regions of the recombinant Fv. The resurfacing of murine monoclonal antibodies may maintain the avidity of the original mouse monoclonal antibody in the reshaped version, because the natural framework-CDR interactions are retained. Again, these antibodies suffer from the problem of being antigenic due to their mouse origins.
Other technologies use primate, rather than mouse, sequences to humanize Mabs. The rationale of this approach, called primatization, is that most of the sequences in the primate antibody variable region are indistinguishable from human sequences. Primatized anti-CD4 Mabs for the treatment of rheumatoid arthritis and severe asthma are being developed. However, these Mabs are still foreign proteins to the immune system of the patient and evoke an immune response.
In an effort to avoid the immune response to foreign proteins, a variety of approaches are being developed to make human Mabs that contain only human antibody components. One approach is to isolate a human B cell clone that naturally makes antibody to the desired antigen and to grow it in a trioma cell culture system. Because human antibodies are made only against antigens that are foreign to the host, none of the human B cells will make antibodies against human antigens. Therefore, this approach is not useful to produce Mabs against antigens that are human proteins.
Two other approaches to create human Mabs are phage display and use of transgenic mice. Phage display technique takes advantage of the ability of humans to make antibodies against any possible structure. This technique uses the antibody genes from many individual humans to create a large library of phage antibodies, each displaying a functional antibody variable domain on its surface. From this library, individual variable domains are selected for their ability to bind to the desired antigen. The Mab is created through molecular biology techniques by combining an antibody variable domain having the desired binding characteristics and a constant domain that best meets the potential human therapeutic product. Again, this technique lacks antigen specificity. The phage library cannot contain every binding region for any and all desired antigens. It also may contain binding regions, which lack specificity. Thus, this technique may require considerable engineering to increase antibody affinities to useful levels.
Transgenic mice are also being used to create “human” antibodies. The transgenic mice are created by replacing mouse immunoglobulin gene loci with human immunoglobulin loci. This approach may provide advantages over phage display technologies because it takes advantages of mouse in vivo affinity maturation machinery.
All of the current technologies for producing human or human-like Mabs are insufficient to provide a species-specific antibody that is antigen specific for a described antigen. Chimeric antibodies have the advantages of retaining the specificity of the murine antibody and stimulating human Fc dependent complement fixation and cell-mediated cytotoxicity. However, the murine variable regions of these chimeric antibodies can still elicit a HAMA response, thereby limiting the value of chimeric antibodies as diagnostic and therapeutic agents.
Vaccines may be directed at any foreign antigen, whether from another organism, a changed cell, or induced foreign attributes in a normal “self” cell. The route of administration of the foreign antigen can help determine the type of immune response generated. For example, delivery of antigens to mucosal surfaces, such as oral inoculation with live polio virus, stimulates the immune system to produce an immune response at the mucosal surface. Injection of antigen into muscle tissue often promotes the production of a long lasting IgG response.
Vaccines may be generally divided into two types, whole and subunit vaccines. Whole vaccines may be produced from viruses or microorganisms which have been inactivated or attenuated or have been killed. Live attenuated vaccines have the advantage of mimicking the natural infection enough to trigger an immune response similar to the response to the wild-type organism. Such vaccines generally provide a high level of protection, especially if administered by a natural route, and some may only require one dose to confer immunity. Another advantage of some attenuated vaccines is that they provide person-to-person passage among members of the population. These advantages, however, are balanced with several disadvantages. Some attenuated vaccines have a limited shelf-life and cannot withstand storage in tropical environments. There is also a possibility that the vaccine will revert to the virulent wild-type of the organism, causing harmful, even life-threatening, illness. The use of attenuated vaccines is contraindicated in immunodeficient states, such as AIDS, and in pregnancy.
Killed vaccines are safer in that they cannot revert to virulence. They are generally more stable during transport and storage and are acceptable for use in immunocompromised patients. However, they are less effective than the live attenuated vaccines, usually requiring more than one dose. Additionally, they do not provide for person-to-person passage among members of the population.
Production of subunit vaccines requires knowledge about the epitopes of the microorganism or cells to which the vaccine should be directed. Other considerations in designing subunit vaccines are the size of the subunit and how well the subunit represents all of the strains of the microorganism or cell. The current focus for development of bacterial vaccines has shifted to the generation of subunit vaccines because of the problems encountered in producing whole bacterial vaccines and the side effects associated with their use. Such vaccines include a typhoid vaccine based upon the Vi capsular polysaccharide and the Hib vaccine to Haemophilus influenzae. 
Because of the safety concerns associated with the use of attenuated vaccines and the low efficacy of killed vaccines, there is a need in the art for compositions and methods that enhance vaccine efficacy. There is also a need in the art for compositions and methods of enhancing the immune system, which stimulate both humoral and cell-mediated responses. There is a further need in the art for the selective adjustment of an immune response and manipulating the various components of the immune system to produce a desired response. Additionally, there is a need for methods and compositions that can accelerate and expand the immune response for a more rapid activation response. There is an increased need for the ability to vaccinate populations, of both humans and animals, with vaccines that provide protection with just one dose.
What is needed are compositions and methods to target the delivery of specific agents to only the target cells. Such compositions and methods should be able to deliver therapeutic agents to the target cells efficiently. What is also needed are compositions and methods that can be used both in in vitro and in vivo systems.
There is also a general need for compositions of monoclonal antibodies and improved methods for producing them. There is a particular need for methods for producing human antibodies having affinity for a predetermined antigen. These human immunoglobulins should be easily and economically produced in a manner suitable for therapeutic and diagnostic formulation.